We have linked our RH-project to RoboSource Control.
With the commandline function, I sould be able to generate my
helpfile automatically and unnattended. But the function does not
seem to work as I hoped it would, because there does not seem to be
a command to get the latest topics from de RSC-database.

Do I miss something here?

And should a RH-client be installed on the PC where I run the
commandline, or can I run it on the database server itself?

The command line generation operates totally independent of
RSC. You point it at the location of your source files and tell it
to run. The command line does not interact with RSC, so it compiles
whatever is in that file set. (This can operate to your advantage
if you want to compile older stuff.)

You'll need to manually update the project for generation
before starting the command line.